The state of the art to which the present invention relates is presented hereinafter in three parts, namely, in relation to: (1) the known technique of mechanical cleaving of optical fibres and optical waveguides, (2) the known techniques for producing lens-shapes on optical fibres, and (3) the known techniques for cutting optical fibres with a laser.
1. Mechanical Cleaving of Optical Fibres and Waveguides
The structure of a typical optical fibre is shown in FIG. 1 of the accompanying drawings. In a number of applications in fibre-optic communications it is necessary to couple light either into or out of optical fibres or waveguides. Sometimes this is accomplished using connectorised fibres where the fibre is attached into a ferrule and then polished to provide an optical quality surface with the end of the fibre lying approximately flush with the end of the ferrule. However, in other applications, the fibre is not connectorised. In these cases, mechanical cleaving of optical fibres is the accepted technique for preparing the ends of the fibre. This is also the case when the ends of the fibre need to be prepared prior to mechanical or fusion splicing.
Mechanical cleaving involves producing a fiducial stress-raising mark on the periphery of the fibre (typically with a diamond blade), and then snapping the fibre from the mark. When carried out correctly, this leaves a high quality, flat surface across the vast majority of the end of the fibre, including across the crucial core region.
In many applications it is required to minimise the optical reflection from the end face of the fibre or waveguide back into the fibre or waveguide. This can be achieved by angling the end face of the fibre or waveguide (as shown in FIG. 1) so that the back-reflected light is reflected away from the core. The greater the angle, the less light is coupled back into the core of the fibre or waveguide. Typically angles of 6–8° are used which are close to the limit of what can be obtained with a degree of reliability in mass production.
In the interests of increasing the component density in opto-electronic devices, however, laser sources which emit vertically (normal to the plane of the chip rather than parallel to it, see FIGS. 2(a) and (b)) are being developed. Coupling the light from these sources into optical fibres or waveguides presents a challenge to conventional techniques, but can be accomplished using total internal reflection form an end face cleaved at approximately 45° to the fibre or waveguide axis as shown in FIG. 2(b).
Mechanical cleaving has a number of disadvantages and limitations. Firstly, it produces very sharp edges on the corner of thee cleaved (cut) fibre. These are susceptible to handling damage, particularly if the fibre is to be inserted longitudinally into another component.
In some cases these sharp edges are removed in a second process, for example by introducing the tip of the fibre into a flame.
Secondly, the range of angles which mechanical cleaving can achieve is limited. Devices relying on stressing the fibre during the cleave process (either by twisting the fibre or by applying a shearing stress) result in an angle on the cleaved end but in practice this is restricted to <10°. Angles of close to 45° required for coupling light into the fibre or waveguide from vertical emitting lasers by means of a reflection from the end face of the fibre or waveguide (see FIG. 2) cannot be achieved. Moreover, the reproducibility of the cleave angle is less than is called for in many applications, with ±0.5° being difficult to maintain in mass production whereas ±0.2° is often desired.
Thirdly, as mechanical cleavers depend for their operation on precision moving parts including a very sharp blade, they are prone to wear and misalignment, requiring more readjustment and refurbishment than is ideal for mass production.
Fourthly, the mechanical cleave process, involving such precise and intimate contact between the cleaver and the fibre, is inherently difficult to automate. Such a non-automated process requires considerable manpower resources to produce large volumes, and the yield is dependent on operator skill which leads to product variability.
Fifthly, the size of the hardware involved in the mechanical cleave means that is not possible to cleave very close to other objects. For example, cleaving cannot generally be carried out closer than about 10 mm from a ferrule or connector.
Further, mechanical cleaving cannot produce the very tight fibre-to-fibre cut length tolerances required of ribbon fibres, where tolerances of ±2 μm or less are required.
2. Producing Lens-Shapes on Optical Fibres and Waveguides
Increasing data traffic is placing ever greater demands on the performance of optical communications systems. These include capacity, bandwidth and distance between amplifiers or repeaters.
Crucial to meeting the above objectives is to maximise the efficiency of the whole system. This not only reduces the power consumed and/or allows the use of fewer amplifier/repeaters, but results in less waste heat and hence thermal loading of the components. This reduces the thermal management hardware needed, permits tighter packaging of components, and allows the active devices to be operated at lower temperatures, which has a significant beneficial effect on component lifetimes.
One significant source of inefficiency in a pig-tailed transmitter or pump laser is the coupling of the emitted laser power into the attached fibre. The problem here is to couple the divergent optical output from the laser diode, which will have an effective source size of a few microns and usually different beam divergences in the two orthogonal dimensions, into the (usually) circularly symmetric core of an optical fibre or waveguide which, for a single mode fibre or waveguide, will be between 3 and 20 μm in diameter, or may be up to 62 μm or more for multi-mode fibre or waveguides.
The optical transfer from the source to the fibre or waveguide is often accomplished using micro-optics inserted between the two components as shown in FIG. 2(c). The production and alignment, assembly and subsequent permanent fixturing of these discrete components is problematic. For reasons of availability and ease of alignment, the lenses are often spherical and symmetric, although it is clear that aspheric, asymmetric lenses would provide superior performance.
Producing a lens-shape directly on the end of the optical fibre or waveguide can reduce the alignment difficulties by avoiding the need for the additional (aligned) component. Various techniques for manufacturing such a lens have been described, including etching, selective etching (where the cladding is selectively removed and the core then etched), grinding, pulling the fibre in the presence of a heat source (usually an electric arc) and laser micro-machining.
The laser route has a number of advantages in terms of speed, flexibility and reproducibility.
The use of a CO2 laser to machine lens shapes on optical fibres by means of a micro-lathe approach has been described in a number of patents (for example, see U.S. Pat. No. 4,710,605, EP 0 391 598 B, EP 0 558 230 B). In these patents, the laser is focused to a spot, which is then scanned across the end of the rotating fibre, providing a machining approach which is analogous to a conventional mechanical lathe.
This approach introduces a significant heat input into the fibre. This results in a re-flow of material which is influenced by surface tension effects. The net result is a smoothing of fine detail and a tendency toward smoothly curved and ultimately near-spherical surfaces. For the purposes of these patents, this is a largely helpful phenomenon when producing relatively gently curved lenses with tip radii (assuming the spherical case) in excess of 10 μm. However, production of radii less than 10 μm is problematic with the micro-lathe technique.
Moreover, in practice the technique is relatively slow (of order 15 s per fibre), and tends to “flare” the fibre, causing the fibre outside diameter (OD) to locally increase beyond the nominal 125 μm, as shown in FIG. 3(a). This is a severe disadvantage if it is wished to passively align the fibre to an active device (say a laser source) by laying the fibre in a v-groove (FIG. 3(b)). In such an application, the tolerance on alignment is typically of order 0.3 μm, and so even 1 μm levels of flare have a significant detrimental effect.
In addition, the significant thermal input in the process described above can result in diffusion of the dopant which defines the core and hence the active region of the fibre (see FIG. 4). This core diffusion can have a deleterious effect on the optical performance of the lens.
Furthermore, the significant thermal input can cause severe problems when machining polarisation maintaining (PM) fibre, which typically have asymmetrically distributed inserts of a different or doped material within the fibre to provide stress directions and hence the PM axis. This different material will generally have different thermal properties to the surrounding quartz, in particular it will melt and re-solidify at a different (usually lower) temperature. If the laser lensing process produces a significant melt region, as the micro-lathe does, the effects of different parts of the end face of the fibre re-solidifying at different times can severely distort the overall surface form.
3. Cutting of Optical Fibres with a Laser
The use of lasers to cut optical fibres has also been described. U.S. Pat. No. 5,421,928 (Siecor Corporation) describes a method in which a focussed laser beam is used to cut excess optical fibre protruding from a ferrule prior to polishing, and EP 0 987 570 A (The Whitaker Corporation) describes a process in which a focussed laser beam is translated across a fibre in order progressively to cut through the fibre (a similar technique is disclosed in U.S. Pat. No. 4,932,989).